Polymer latexes are typically stabilized by external anionic surfactants. Stability is often further enhanced by the inclusion of structural units in the polymer of carboxylic acid or carboxylate groups arising from the polymerization of monomers such as acrylic acid, methacrylic acid, and itaconic acid and salts thereof. Although the use of these acid-containing monomers is considered essential for commercial preparation of latexes, finding the optimal concentration is critical: Too much acid monomer results in film blistering, increased associative thickener demand, and poor resistance to hydrophilic solvents. Too little causes instability of the latex. As such, latexes commonly used in industrial applications contain the least amount of acid monomer necessary to impart stability to the particles.
Young has reported that incorporating epoxy resins into latex particles improves “handling performance and wet and chemical strength advantages over unmodified latex formulations” while reducing or even eliminating the need for volatile organic compounds (VOCs). (Young, G. C., “Modifying Latex Emulsions with Epoxy Resin Dispersions”, Adhesives Age, pp. 24-27, (1996)). Nevertheless, the stabilizing advantages of carboxylic acid salts are offset by their reactivity toward these thermosettable compounds. However, the reactivity of these thermosettable compounds with carboxylate salts causes latex particle instability. Young suggests that the reactivity of the acid group can be suppressed by lowering the pH of the polymer prior introduction of the epoxy resin; however, a skilled artisan would recognize that colloidal stability of the latex would be compromised at low pH. Significantly, Young only teaches that up to 20% of the epoxy resin can be incorporated into the latex particle (with examples only up to 10% incorporation) and makes no mention of the percent solids of the latex with incorporated epoxy resin.
Consequently, it remains a challenge to imbibe more than 10% of a thermosettable compound into a high-solids (40-60 weight percent) polymer particle dispersion and maintain particle stability under standard industry protocols, namely heat-age stability testing. It would therefore be an advance in the art to discover a heat-age stable curable latex 2-pack system that allows for the incorporation of relatively high concentrations of a thermosettable compound.